In a color television receiver having a picture tube with cathodes for producing three electron beams for the basic colors red, green and blue, it is known to control the beam emissions by means of the three chrominance signals R, G and B. In order to obtain a perfect color picture, in particular with respect to the correct combination of the three colors, it is necessary to adjust the chrominance signals in various ways.
Initially, the chrominance signals must be adjusted with respect to their d.c. level so that their black value will be applied to the control electrode of the picture tube at that voltage value at which the beam current just begins to flow, i.e., the black value of the chrominance signal must be adjusted to the point of beam current actuation, the so-called cut-off point of the picture tube. This setting must be made individually for all three color systems and differs from picture tube to picture tube because the cut-off points fluctuate from one set to another. This setting is necessary so that the picture tube will still be dark controlled at the black signal level.
Furthermore, the amplification of the three chrominance signals must be matched correctly with one another so that all three colors will have the same intensity on the screen. This setting is necessary particularly to adapt the entirety of the circuitry to the characteristics of the picture tube and to the efficiency of the phosphors of the screen. It indicates the relationship between the beam current of a system and the brightness of the respective color produced thereby on the screen.
In the prior art color picture tubes which employ three electron guns disposed at the corners of an equilateral triangle and three separate screen grids for the three beam systems, the matching of the direct voltage position of the video signal to the cut-off points is effected by individual setting of the screen grid voltages. The setting of the gain is effected by means of two adjusters in the amplifiers for two of the chrominance signals. With these adjusters the amplification factors are set so that the brightness intensity of the three colors on the screen will be the same.
In the so-called precision-in-line (PIL) color picture tubes in which the three electron guns are not disposed at the corners of an equilateral triangle but in a horizontal plane, the three screen grids of the picture tube are connected together. Individual setting of the beam current starting point at the screen grids is thus impossible. It is therefore necessary to provide means for matching the direct voltage level of the chrominance signals to the beam current starting point in the three chrominance signal paths. Shifting of the beam current starting point in the picture tube also changes the required amplification for the chrominance signal due to the characteristics of the picture tube. Thus it is additionally necessary to provide a total of three setting possibilities for the amplification factors in the three chrominance signal amplifiers.